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  #1  
Old June 26th 19, 04:11 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
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Posts: 87
Default green led

My computer case green led failed. I bought some on Ebay, but they are
so bright they light up the room and obscure the other color leds.

Any suggestions for a cheap source of low intensity leds in a 3mm size?
Maybe Mouser, but then $1 worth of parts costs $10 to ship. If I have to
spend that much to fix it, I'll just live without a power led.

Can you add a resistor or something to change the brightness?


  #2  
Old June 26th 19, 04:33 PM posted to alt.comp.hardware.pc-homebuilt
Banders
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Posts: 2
Default green led

On 06/26/2019 08:11 AM, T. Ment wrote:
My computer case green led failed. I bought some on Ebay, but they
are so bright they light up the room and obscure the other color
leds.

Any suggestions for a cheap source of low intensity leds in a 3mm
size? Maybe Mouser, but then $1 worth of parts costs $10 to ship. If
I have to spend that much to fix it, I'll just live without a power
led.

Can you add a resistor or something to change the brightness?


My truck has a manual transmission, and a green LED in the instrument
panel that tells me when it's time to shift up into the next gear. It
was brilliant, and usually wrong. A black Sharpie pen fixed it. I can
barely see it now, even at night.
The blue Hi-Beam indicator LED was also dazzling. About 70% coverage
with Sharpie toned it down.
  #3  
Old June 26th 19, 06:41 PM posted to alt.comp.hardware.pc-homebuilt
Paul[_28_]
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Posts: 1,467
Default green led

T. Ment wrote:
My computer case green led failed. I bought some on Ebay, but they are
so bright they light up the room and obscure the other color leds.

Any suggestions for a cheap source of low intensity leds in a 3mm size?
Maybe Mouser, but then $1 worth of parts costs $10 to ship. If I have to
spend that much to fix it, I'll just live without a power led.

Can you add a resistor or something to change the brightness?


Yes, you can add a resistor.

The case wiring has a harness with twisted pairs. There
is always some series resistance in the path to limit
the current. (Because making a precise regulated voltage
to drive the LED with a voltage source would cost too much
to do.) The resistor helps limit the current flow. 20mA
is a typical design value for operating current for T1 3/4
LEDs (or so). YMMV. Check datasheet. The following diagram
is simplified so only the concept is present.

Mobo +5V ---- Rseries ------------------------- LED-FP
GND---------------------------------------

You can either pull the panel assembly from the case
(could be a small PCB that is removable) and add
an additional resistor there.

In any case, you want this.

Mobo +5V ---- Rseries ------------- Rseries --- LED-FP
GND---------------------------------------

If you're brave, you could certainly change the
resistor on the motherboard, but it would be SMT
and you'll need skillz to remove. (I use two
soldering irons for this, say a 25W and a 15W,
heat the ends of the SMT and move it out of the way.
Rotating the soldering irons as you pull them away
from the work, helps prevent the SMT from "sticking"
to the tip of the iron.)

Putting the resistor in the wiring harness
would work. You need shrink wrap (Polyolefin tubing)
to do a neat job and insulate the work. Electrical
tape is a no-no (a gooey mess).

I'd try and do it at the front panel PCB, so the
job is neat and tidy.

*******

On a computer case here, I taped a piece of "white card",
like a recipe card, over the blue power LED on the
computer, and that provides sufficient attenuation
to make the computer case "eye safe", if you know
what I mean. Now you don't need a soldering iron.

Paul
  #4  
Old June 26th 19, 07:08 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
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Posts: 87
Default green led

On Wed, 26 Jun 2019 13:41:49 -0400, Paul wrote:

Can you add a resistor or something to change the brightness?


Yes, you can add a resistor.

The case wiring has a harness with twisted pairs. There
is always some series resistance in the path to limit
the current. (Because making a precise regulated voltage
to drive the LED with a voltage source would cost too much
to do.) The resistor helps limit the current flow. 20mA
is a typical design value for operating current for T1 3/4
LEDs (or so). YMMV. Check datasheet.


What datasheet. I bought a pack of them on Ebay for $1.50, postage
included.


The following diagram
is simplified so only the concept is present.

Mobo +5V ---- Rseries ------------------------- LED-FP
GND---------------------------------------

You can either pull the panel assembly from the case
(could be a small PCB that is removable) and add
an additional resistor there.


There's no PCB in the front panel. The wires run from the motherboard
directly to the LED, using small connectors at the LED housing. I will
have to cut the positive side and patch in a resistor.



In any case, you want this.

Mobo +5V ---- Rseries ------------- Rseries --- LED-FP
GND---------------------------------------


First you had one Rseries, now two. I don't get it.



If you're brave, you could certainly change the
resistor on the motherboard, but it would be SMT
and you'll need skillz to remove.


I have some skills, but I won't risk a motherboard to fix the computer
case LED.


Putting the resistor in the wiring harness
would work. You need shrink wrap (Polyolefin tubing)
to do a neat job and insulate the work. Electrical
tape is a no-no (a gooey mess).

I'd try and do it at the front panel PCB, so the
job is neat and tidy.


That's what I'm thinking. But I don't know what resistor value to try.
Any ideas?




  #5  
Old June 26th 19, 09:25 PM posted to alt.comp.hardware.pc-homebuilt
Paul[_28_]
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Posts: 1,467
Default green led

T. Ment wrote:

In any case, you want this.

Mobo +5V ---- Rseries ------------- Rseries --- LED-FP
GND---------------------------------------


First you had one Rseries, now two. I don't get it.


You add additional series resistance, to reduce the current flow.


That's what I'm thinking. But I don't know what resistor value to try.
Any ideas?


It's too bad you didn't mention the Ebay item, to see
if there was any semblance of a specification for it.

They have a lens (15 degree to 75 degree). The tighter
lens means the LED cannot be seen off to the side all
that well, but the beam in the center would be blinding.

The computer case assumes about 20mA. Lets try some
sample math to demonstrate the basics.

1.6V @ 20mA

+ |\ | -
+5V ------ Rseries ------} |------+
|/ | |
|
ground (black)

Each color of LED has a forward voltage drop. The blue
LEDs might be 2.5V. A red LED might be 1.6V. We'll use
a red LED for this exercise.

I want to work out an RSeries value.
The LED has a recommended current of 20mA.

5V - 1.6V 3.4V
----------- = Rseries_ohms = ------ = 170 ohms
0.020 amps 0.020A

Maybe the nearest 5% carbon composition 1/4W resistor to
that is 180 ohms. Let's check the resistor power to
make sure the resistor won't burn.

P = I^2*R = 0.02 * 0.02 * 180 = 72mW is less than 250mW (1/4 watt)

so my choice of a quarter watt resistor is OK.

Now, eyeball the LED and ask yourself "what integer factor
do I want to reduce the brilliance?"

Let's say it's really bright, so I'll reduce by a
factor of five. I want to go from 20mA to 4mA.
The Vf varies with If, but we'll ignore this and
do our math again.

5V - 1.6V
----------- = 850 ohms
0.004 amps

Nearest 5% resistor is 820 ohms.

OK, let's assume there is a 180 ohm resistor
already in the circuit, and, I want a total
of 820 ohms in the path. Take 850 - 180 = 670 ohms,
and a 680 is the nearest 5% resistor. My proposed
circuit becomes, like this. We're playing "close enough"
here, and all maths are only done to get in the right
ballpark. This isn't a "reference grade circuit" or
something, requiring 1% resistors and careful work.

180ohm 680ohm
Mobo +5V ---- Rseries ------------- Rseries --- LED-FP
GND---------------------------------------
(on Mobo) (added R)
(in harness)

That's how you do it.

If we assume 200 lumens/watt, then 1 lumen would be
5mW, or 1.6V*3mA or so. Just to try to put this into
some sort of perspective. I think the lumen rating is
total integrated optical power over the entire
lighting surface. And the 200 number is the typical
efficiency of at least blue LEDs today. The ones I own
are in that ballpark.

This is why the datasheet can give us some ideas on
what we bought, and "whether it'll take out an eyeball".

I don't find the MCD (milli-candela???) rating all that
useful, as I think that's a peak power at zero degrees
to the axis of viewing. A narrow lens elevates that power
and doesn't really tell you what it looks like, all that
well.

*******

And this is the way to buy resistors. An "assortment pack"
gives resistors at from $0.03 to $0.05 a piece. This one
happens to be sold out, but it's just an illustration.

https://www.radioshack.com/collectio...nt=20332255301

You will find 30 each of 1k, 10k, and 100k;
ten each of 1, 10, 100, 120, 150, 220,
330, 470, 560, 1.5k, 2.2k,
3.3k, 4.7k, 15k, 22k, 47k,
220k, 470k, 1meg, and 10meg;

Five each of 2.2, 15, 22, 33, 39, 51, 68, 82,
180, 270, 390, 510, 680, 820, 1.2k,
1.8k, 2.7k, 3k, 3.9k, 5.1k, 5.6k,
6.8k, 8.2k, 12k, 18k, 27k, 33k, 39k,
51k, 56k, 68k, 82k, 120k, 150k, 180k,
270k, 330k, 1.5meg, 2.2meg, 3.3meg,
and 4.7meg.

Notice I'm getting 180, 680, 820, the usual kinds
of standard values.

If you buy resistors in one-pack or two-pack, sometimes
they charge a buck a package. Which is outrageous.
(And imagine the shipping, if you have no electronics
store in town.)

Now, the assortment pack I used to be able to get, costs
$2.50 a pack and is "like Noahs Ark", as you get two
resistors of each value. And that's an excellent intro
to filling up your resistor trays. While you might
easily "run out" using such assortments, you'll get
a good selection of "trial" resistors to put in
your circuit.

*******

A potentiometer allows adjusting the resistance, which
is a variable resistance device. With one of those in
the circuit, you would dial just the right light level.
So it is possible to do a more fancy job. This is a 1K
variable. I wouldn't do it this way though. I don't
waste pots on stuff like this. Single resistors are
sufficient.

https://www.digikey.ca/product-detai...ducti d=84508

*******

One other thing you should know. The T 1 3/4 LEDs you're
working with, are "reverse polarity safe". If you plug it
in backwards, it's not damaged. They happen to be rated
for 5V PIV (Peak Inverse Volts). That just happens to
make them "almost hobbyist safe". There are some
more expensive LEDs, for lighting purposes, with
tremendous power rating, that can be "gimped" if you
plug them in backwards. Again, if you buy the expensive
ones, get the datasheet first and review the safety info.
They'll tell you if there is a reversal problem or not.

The LED has legs of uneven length. The plastic
body has a "flat" on one edge. These help predict
the polarity, and which side is the anode and which
side is the cathode.

*******

Sample datasheet:

https://www.cree.com/led-components/...S-ACN-1079.pdf

Amber colored

2.1V @ 20mA

15 degree lens (30 degree cone)

Some are 12000 MCD (binned products)

Page 10 shows how the polarity indicators work (leg length, the flat)

HTH,
Paul
  #6  
Old June 26th 19, 09:41 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default green led

On Wed, 26 Jun 2019 16:25:17 -0400, Paul wrote:

It's too bad you didn't mention the Ebay item, to see
if there was any semblance of a specification for it.


https://www.ebay.com/itm/382885459517

They say:


Max. Forward Current: 20mA

- For red ,yellow ,green it is 2V , 20mA ,forward voltage is 2-2.2V


Maybe I'm overvolting it. I think that motherboard pin had 5v. But I'm
not sure. I need to open the case and check it again. I'll get back to
you later on that.




  #7  
Old June 26th 19, 10:40 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default green led

On Wed, 26 Jun 2019 20:41:14 +0000, T. Ment wrote:


Max. Forward Current: 20mA

- For red ,yellow ,green it is 2V , 20mA ,forward voltage is 2-2.2V


Maybe I'm overvolting it. I think that motherboard pin had 5v. But I'm
not sure. I need to open the case and check it again. I'll get back to
you later on that.


I remembered it right, I have 4.85v on that pin. Maybe the part I bought
won't work, since they say 2v above.

Your post shows 5v input. Not sure I can understand all that, but I will
read it again.



  #8  
Old June 26th 19, 10:44 PM posted to alt.comp.hardware.pc-homebuilt
Paul[_28_]
external usenet poster
 
Posts: 1,467
Default green led

T. Ment wrote:
On Wed, 26 Jun 2019 16:25:17 -0400, Paul wrote:

It's too bad you didn't mention the Ebay item, to see
if there was any semblance of a specification for it.


https://www.ebay.com/itm/382885459517

They say:


Max. Forward Current: 20mA

- For red ,yellow ,green it is 2V , 20mA ,forward voltage is 2-2.2V


Maybe I'm overvolting it. I think that motherboard pin had 5v. But I'm
not sure. I need to open the case and check it again. I'll get back to
you later on that.


Obiwan, use the maths.

Plug in the 2.2V instead of the 1.6V in my example.

Select a current flow, less than 20mA, like 4mA, and
work out the 1/4W resistor to use.

5V - 2.2V 2.8V
----------- = Rseries_ohms = ------ = 700 ohms
0.004 amps 0.004A

If there is 180 ohms already on the motherboard, we need
another 520 ohms. Look in the assortment pack I showed,
for the nearest higher value compared to our calc
value of 700 minus the 180 already on the motherboard.

"Increasing resistance = Less light"

*You can only hurt the LED, if you plug direct to +5V*

With some resistance in the circuit, and not exceeding
the PIV if you were to plug it in backwards, you're
made in the shade.

____ new-Rseries _____
\ /\ / +
x x LED
/ \/ \______________________ -

twisted
pair
harness

The resistor needs to be series-connected to limit
the current flow and reduce LED intensity.

Paul
  #9  
Old June 26th 19, 10:58 PM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default green led

On Wed, 26 Jun 2019 17:44:20 -0400, Paul wrote:

Obiwan, use the maths.

Plug in the 2.2V instead of the 1.6V in my example.
The resistor needs to be series-connected to limit
the current flow and reduce LED intensity.


I get the math. But the terminology confuses me. Like forward voltage
drop. What about maximum input voltage. There must be a limit before you
burn it up.


  #10  
Old June 27th 19, 12:13 AM posted to alt.comp.hardware.pc-homebuilt
T. Ment
external usenet poster
 
Posts: 87
Default green led

On Wed, 26 Jun 2019 21:58:03 +0000, T. Ment wrote:

I get the math. But the terminology confuses me.


No doubt the circuit is simple to you. But my electronics knowledge is
very sparse.

I know a resistor "resists" but what does it reduce? Current, voltage,
or both?

Where is the voltage dropping? At the resistor? At the LED? Or both?


 




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